(1) Field of the Invention
The present invention relates to a method of manufacturing a semi-hard magnetic material, for example used as a bias material for a crime prevention sensor, and to a semi-hard magnetic material.
(2) Description of Related Art
A magnetic sensor tag attached to goods at large-sized mass merchandisers or the like for preventing burglary (which is referred to as a “crime prevention sensor” hereinafter) is composed of a resonating magnetostrictive strip and a bias which gives a magnetic field thereto.
The system of the crime prevention sensor has a function that the magnetostrictive strip resonates and an alarm sounds when somebody attempts to take a product out of a shop without paying a charge. When a charge is duly paid, however, the resonance frequency is necessary to be changed so as not cause the magnetostrictive strip to resonate. To change the resonance frequency of the magnetostrictive strip, it is necessary to change the intensity of the magnetic field that the bias gives to the magnetostrictive strip. More specifically, the bias needs to remain in a magnetized state until a charge is paid, but be changed to a demagnetized state after the charge is paid.
For this reason, after the payment, an operation of demagnetizing the bias is performed using a demagnetization apparatus mounted on a counter stand of a register. In this case, if the coercive force of the material which composes the bias is too large, it is difficult to realize demagnetization. On the contrary, if the coercive force is too low, it is easy to realize demagnetization, but there is a problem that the magnetic field given to the magnetostrictive strip in the magnetized state becomes small. Furthermore, when a tiny reverse magnetic field is applied to the bias, the function as the crime prevention sensor is lost and this deteriorates the reliability.
As the material for the above described bias, a semi-hard magnetic material which has an intermediate coercive force between a hard magnetic material which has a high coercive force of not less than 8000 A/m (permanent magnet) and a soft magnetic material which has a low coercive force of not more than 800 A/m is preferably used.
More specifically the hard magnetic material preferably has a coercive force Hc of 1000 to 5600 A/m, more preferably in a range of 1200 to 4000 A/m, and preferably it has a remarkable difference between ON and OFF, that is in a magnetized stats and a demagnetized state. Therefore, the hard magnetic material preferably has magnetic characteristics which include a high saturation magnetic flux density Bs and residual magnetic flux density Br as well as a high squareness ratio Br/Bs on a B—H curve.
The semi-hard magnetic material may be also used for a relay or motor in addition to the above described crime prevention sensor.
As one of such semi-hard magnetic materials, JP-A-60-116109 discloses an Fe—Ni—Mo semi-hard magnetic material composed of 16.0 to 30.0% of Ni, 3.0 to 10.0% of Mo and the balance being substantially Fe in mass % and a manufacturing method thereof.
According to this document, rolling, drawing and swaging processes at a reduction ratio of 20 to 80% are performed. As for a metallic structure then, a martensitic structure increases according to a degree of work and transforms into two-phase structure of austenitic and martensitic structure. The document further discloses processes of holding the austenitic and martensitic structures at a temperature of 600 to 700° C. for 10 minutes to 5 hours for generating reverse transformed austenite to transform it into a mixed structure of 30 to 70% of austenitic structure and martensitic structure, then reworking it at a reduction of 50 to 98%, and then subjecting to final ageing by holding it at a temperature of 500 to 600° C. for 10 minutes to 5 hours to generate a reverse transformed austenitic structure for adjusting it to have 30 to 70% of austenitic structure in mass %.
On the other hand, JP-A-2000-504069 proposes a method of manufacturing an Fe—Ni—Mo semi-hard magnetic material composed of 16.0 to 30.0% of Ni, 3.0 to 10.0% of Mo, and the balance being substantially Fe in mass %, comprising heating the material of a martensitic structure at approximately 475 to 625° C. for approximately 4 minutes to generate reverse transformed austenite, and then cold-rolling it to extend the reverse transformed austenitic structure into an extended structure so as to obtain a desired coercive force (not less than 2400 A/m).
In the above two proposals, reverse transformed austenite is intentionally generated even in an intermediate process to obtain a mixed structure of martensitic and austenitic structures, and thus a final mixed structure of martensitic and austenitic structure is obtained.
According to JP-A-60-116109, in order to generate 30 to 70% austenitic structure in the martensitic structure, the mixed structure of the austenitic and martensitic structures is kept in each process and finally adjusted to a desired metallic structure through ageing. However, the metallic structure before the final ageing is apt to change in each process, for example due to its thermal history, and the metallic structure is further changed by a rolling reduction in each path of cold rolling, therefore, a fixed final ageing condition may cause a variation in the magnetic characteristics.
Furthermore, according to JP-A-2000-504069, the generated reverse transformed austenite is cold rolled and transformed into an extended structure of reverse transformed austenite so that a layered structure of martensite and reverse transformed austenite is obtained. However, according to present inventor's investigations, when the reverse transformed austenitic structure is subjected to cold rolling, it transforms into martensite at a minimal rolling reduction and the amount of transformation thereof also changes depending on the temperature of rolls which contact the material (rolling temperature). Therefore, the technique of adjusting the amount of reverse transformed austenite using the cold rolling process as the final process requires a high-level manufacturing technique.
In view of the above described problems, it is an object of the present invention to provide a method of manufacturing a semi-hard magnetic material capable of efficient industrial production with relatively easy adjustment of an amount of reverse transformed austenitic structure. Furthermore, it is another object of the present invention to provide a semi-hard magnetic material which has a desired material structure and magnetic properties, such as coercive force and squareness ratio.